Previous research in developmental behavioral genetics of cognitive abilities has focused on traditional psychometric assessments rather than domains and processes of current interest in cognitive psychology. One such domain is the control and coordination of specific cognitive processes during the performance of complex tasks. These postulated executive functions are general-purpose control mechanisms, neurologically associated with frontal lobe activity, that modulate the operation of various cognitive subprocesses and thereby regulate the dynamics of human cognition. The goal of the present proposal is to conduct the first behavioral genetic study of individual differences in executive functions in a genetically informative twin sample already characterized for general and specific cognitive abilities. We will focus on the three best supported components of executive function: shifting of mental sets ('Shifting'), monitoring and updating of working memory representations ('Updating'), and inhibition of prepotent responses ('Inhibition') as well as the most widely used 'frontal lobe' or executive tasks. The proposed research will: (1) Confirm (or reject), in a sample from the general population showing a full normal range of IQ, the phenotypic correlations and factor structure for executive function components and tasks previously observed in selected samples; (2) assess the genetic and environmental contributions to executive function components and tasks, estimating both the heritable and environmental variance of each and the genetic and environmentally influenced correlations among them; (3) determine, through multivariate genetic and environmental confirmatory facto analyses, whether individual differences in executive functions reflect common genetic or environmental factors across components and tasks or, alternatively, manifest a high degree of component or task specificity; and (4) determine the extent to which individual differences in executive functions are genetically or environmentally independent of traditional conceptualizations of intelligence. We will test 644 individual twins, from 172 monozygotic and 150 dizygotic pairs, who have participated in the Colorado Longitudinal Twin Study since infancy. These subjects will be tested at age 17 years on three index tests of each of the three components of executive functioning, three widely used executive function tasks (the Wisconsin Card Sort Test, the Operations Span test, and the Tower of Hanoi), and two tests of fluid intelligence (Raven's Progressive Matrices Test and Cattell's Culture Fair Test). Data will be subjected to confirmatory factor analyses multivariate phenotypic analyses, and then genetic and environmental structural equation analyses to address the four specific aims.